1. Field of the Invention
The present invention is concerned with an improved, high speed screen printing machine which permits extremely accurate imprinting of successively fed planar segments. More particularly, the invention pertains to such a screen printing machine, and corresponding methods, wherein a camera is located adjacent the bottom face of the segment to be imprinted and is oriented to view a marginal segment indicium through the segment material while simultaneously viewing a reference indicium carried on the screen; this combined image data is then used to adjust the screen, preferably by simultaneously moving the screen along two transverse adjustment axes parallel to the plane of the segment, and about a rotational axis transverse to such plane.
2. Description of the Prior Art
It is known to provide a screen printing press for printing on a web having defined areas or segments that are successively brought into a position to be printed. The press includes a flat, image-bearing screen supported on the base of the press for movement between a printing position overlying the web and an interrupted position spaced from the web. In addition, a web handling assembly is provided for advancing the web along a path of travel to successively feed the defined segments toward a work station defined by the press base.
When the web is to undergo multiple printing processes, e.g. in the production of multi-color images, it is necessary to print each individual color on the web and to dry the web prior to printing the subsequent color. Therefore, as the web is fed to the work station for the second and all subsequent printing operations, it is necessary to register each successive image, or defined segment, with the screen so that all of the colors are in register with one another. Typically, such registration is achieved manually by adjusting the position of the screen on the press and printing proof images on the web until registration is obtained. Thereafter, it is assumed that the web and screen remain in register as each successive area of the web is fed a predetermined distance to the work station by the web handling system. If misregistration occurs due to stretching or contraction of the web material, the press is stopped and the screen position is readjusted to properly register with the web. Alternately, it is possible to manually adjust the incremental feed of the web to compensate for such misregistration.
It is also known in the screen printing art that the centerline of the image to be imprinted should be located in registry with the centerline of the web for optimum printing results. If only a single web width is employed, such centerline matching does not present any significant problems. However, where a given screen printing machine is designed to handle webs of varying widths, a problem arises in that it is difficult to rapidly and accurately realign the machine for varying web widths while still maintaining centerline printing.
Pending application for U.S. patent Ser. No. 08/658,025 describes a screen printing press having longitudinal, lateral, and angular screen frame registration. In the described press, longitudinal adjusting means is provided for precise longitudinal alignment between the screen and web along the X axis (i.e., along the path of web travel). A separate adjusting mechanism is used for lateral (Y axis) and angular (.theta. axis) adjustments between the screen and web. Accordingly, the press of the '025 application is incapable of simultaneous X, Y, .theta. adjustments.
Pending application for U.S. patent Ser. No. 08/825,368 describes a web processing apparatus such as a die cutting machine which includes a unique three-motor adjustment mechanism permitting simultaneous X, Y, .theta. adjustment of a segment-holding platen. Both of the aforementioned U.S. patent applications are incorporated by reference herein.